I've owned 3 pairs of Dynaudio speakers, Special 25s, Contour 1.3SE and the Contour 2.8 (with the very nice Esotar tweeter). I liked and appreciated them all, even though I've moved on from them. At one time I owned them all at the same time, although the 1.3SEs were the first to go.
In some ways, their type of sound simply isn't like any pair of headphones because of the very different sort of sound staging you get from speakers. A good pair of two way stand mounts have such good sound staging it can be spooky, something headphones can't match - but of course they have their own special skills.
But otherwise if I had to pick, I would agree, of the headphones I've listened to I'd liken them to HD 650s. They have that same sort of sound signature that makes them so easy to listen to for extended periods of time, and one other common characteristic - you want to keep turn the volume up until they hit that "come alive" level at which point they are still easy to listen to for extended periods of time. Lesser equipment gets harsh or bright or fatiguing - not initially but after a while. In the speaker world I experienced this with the B&W Nautilus line, and there are many bright and fatiguing headphones, especially if you are sensitive to ragged treble and are bothered by harshness in the upper registers. I enjoyed the bass on all the Dyns, the Special 25s are just phenomenal for bass considering that they are two way stand-mounts, but only when driven by good high current amps (IMO). The 650s are similar again, they need to be driven with authority by a good amp.